The point is just about goal scoring. Afonso Alves had a good scoring record in the Swedish league, an incredible record in the Dutch league, an average at best record in England and pretty good in Qatar. That's not the record of a fabulous player.Afonso Alves was a fabulous player. It's a shame that it went pear shaped for him at Boro.
Of course there are some players who've continued scoring after leaving the Dutch league (fat Ronaldo, RvN, Romario) and others who've been successes after leaving while still not scoring as many goals (Bergkamp and Kuyt say). The point that's being made is that even a ridiculous scoring record in the Netherlands doesn't mean a player will score goals in another league.